Temperature sensors are used in a wide variety of applications across many different disciplines. For example, high temperature resistance temperature detector (RTD) sensors are used in emission-control systems used with internal combustion engines because they can detect changes in temperature with time constants on the order of about 10 seconds or less as is required in that type of application. However, not all temperature sensors are suitable for use in emission control systems.
To measure temperatures up to about 1,000xc2x0 C., thermocouples that use platinum/platinum-rhodium or nickel/chromium-nickel or the like have been used. These devices however, are prone to becoming poisoned when exposed to an exhaust gas at high temperature; they also have relatively long response times to temperature changes, since they must be of a large diameter for durability purposes.
To improve the response times of RTD sensors, temperature sensors comprising platinum group metals have been screen-printed or produced using other thin-film technologies onto various ceramic substrates. While the response times of such thin film sensors are improved compared to thermocouples, such thin-film sensors are only suitable for use at temperatures up to a maximum of about 850xc2x0 C., wherein these sensors become unstable during operation. The stability of a sensing element may also be affected by poisoning of the sensing element like that which can occur when a platinum sensing element comes in contact with materials such as silicon, lead, and the like, all of which may be present in exhaust gases from an internal combustion engine.
Approaches directed to increasing the maximum temperature at which a thin film platinum group metal temperature sensor may be used include protecting the temperature sensor from oxidation and other effects by covering the sensor element with a protective cover plate, also referred to as a protective layer or passivation layer. A cover plate may comprise a single layer, or multiple layers of a heat-resistant ceramic material. Examples include GB 2 171 253, which is directed to a temperature sensor made of platinum metal that is provided with an aluminum oxide protective layer. JP-A 63269502 discloses a platinum resistance film having a silicon nitride protective layer. German Offenlegungsschrift 36 03 757 is directed to protecting a platinum sensor with a titanium dioxide/silicon dioxide double layer.
Also, to provide protection of the sensing element, the cover plate can be hermetically sealed from the exhaust gas or other harmful environments. By hermetically sealed it is meant that the sensing element is encased behind the cover plate so that an essentially impermeable barrier is formed between the sensing element and the environment. If there is a leak that allows contact between the exhaust gas and the sensing element, even through a small pinhole, poisons contained within the exhaust gas can seep in and change the response characteristics of the sensing element causing instability.
Another approach of providing an impermeable seal around a sensing element is disclosed in U.S. Pat. No. 5,430,428 to Gerblinger et al., wherein a double layer passivation layer is disposed over a platinum resistive layer (i.e., a sensing element) to prevent oxidation of the element. The double layer includes a ceramic layer (i.e., a cover plate) and a glass layer that are both disposed directly over a substrate to which the sensing element is attached. As such, the sensing element is encased within a layer of glass between a substrate and a cover layer. This approach, however, can allow poisons to diffuse through the glass to the sensing element under high temperature conditions wherein the glass becomes permeable to poisons present in exhaust gases.
Another approach to providing an impermeable seal around a sensing element is to provide a glass seal between a substrate and a cover plate around the sides of the cover plate and the corresponding sides of the substrate on which the sensing element is disposed. However, this approach can interrupt or sever physical and thermal contact between the cover plate and the sensing element, which increases response times of the sensor. Also, to attach only the edges of the cover plate to only the edges of the substrate, the two pieces are held together while a bead of glass slurry is disposed onto the edges of both the substrate and cover plate, both being of equal dimension. Heating followed by cooling causes the glass to melt and then harden, thereby sealing the cover plate to the sensing element substrate. Difficulties experienced while applying glass to the sides of the element without the various parts moving during the application have limited the effectiveness of the this approach.
Furthermore, in this and similar approaches, gravity can cause the glass slurry to become deformed and flow prior to it being fired and then cooled into a solid. Thus, if the element is positioned such that the side where the glass slurry is being applied is horizontal (so as not to be affected by gravity), the element must be positioned and then repositioned prior to application of the glass slurry to each side of the cover plate-substrate interface. Such manipulation during the process leads to an increase in process time and complexity.
Another attempt at protecting the sensing element from a harmful environment includes packaging the entire RTD temperature sensor within a protective sheath which is vented to the atmosphere. However, such an approach is detrimental to response time, often resulting in a response time three to four times longer than the same temperature sensor without the protective sheath.
Accordingly, there remains a need in the art for a stable temperature sensor having relatively fast response times that is protected from poisons. This need includes a temperature sensor that can be produced without undue complexity and processing time.
Disclosed herein is a temperature sensor and methods of making and using the same. In one embodiment, the temperature sensor comprises: a cover plate disposed at a first end of a substrate to form an interface portion; a sensing element disposed between the cover plate and the substrate to form an assembly; wherein the cover plate and substrate have relative dimensions so as to form a ledge at the first end; and wherein the cover plate is attached to the substrate at the ledge.
In another embodiment, the temperature sensor comprises: a cover plate disposed at a first end of a substrate to form an interface portion, a sensing element disposed between the cover plate and the assembly, and a seal disposed to inhibit fluid communication between the sensing element a gas to be temperature sensed, wherein fluid communication is retained between the sensing element and an external environment.
In one embodiment, the method of making a temperature sensor comprises: disposing a cover plate over a first end of a substrate comprising a sensing element such that the sensing element is located between the substrate and the cover plate to form an assembly, wherein the cover plate and substrate have relative dimensions so as to form a ledge at the first end, and attaching the cover plate to the substrate at the ledge.
In another embodiment, the method of making a temperature sensor comprises: disposing a cover plate over a first end of a substrate comprising a sensing element such that the sensing element is located between the substrate and the cover plate, and attaching the cover plate to the substrate to form a seal that is capable of inhibiting fluid communication between the sensing element and a gas to be temperature sensed, wherein the sensing element is in fluid communication with an external environment.
The above described and other features are exemplified by the following figures and detailed description.